Good troubleshooting so far. There are a couple things I would try since you’ve already tried so much already.
1) you mention being on iOS 12. Are you on the latest update? 12.1.2?
2) This will sound odd but it often relates to a lot of connection problems. Go to Settings > General > Date and Time > turn Set Automatically off, wait a few seconds, then turn it back on. If it’s already off, turn it on.
3) Go to settings > cellular > wait until the list of apps load, scroll down to Kik, then turn that switch off, then back on.
4) Last thing to try would be to reset your network settings. You can do this by going to Settings > General > Reset > Reset network settings. You will have to reconnect to your saved Wi-Fi networks and some bluetooth devices.
If none of those work, comment on this and Ill give you some more stuff to do next.